Masten Gregory was an American racing driver. He raced in Formula One between 1957 and 1965, participating in 43 World Championship races, and numerous non-Championship races.

Known as the "Kansas City Flash", Masten Gregory was born in Kansas City, Missouri as the youngest of three children; his elder brother was Riddelle L. Gregory Jr., also a race car driver, and his elder sister Nancy Lee Gregory married, as her second husband, the Anglo-American fashion designer Charles James. An heir to an insurance company fortune, Gregory was well known for his youngish looks and thick eyeglasses, due to his "terrible" eyesight. Although he attended the Pembroke-Country Day School in Kansas City, he left school before completing his senior year, and married Luella Simpson at the age of 19. His parents divorced when he was very young, and his father died when he was three years old. As an adult, Gregory used his inheritance to buy a Mercury-powered Allard, which he drove in his first race, the 50-mile (80 km) SCCA race in Caddo Mills, Texas in November 1952. He retired from that race due to head gasket failure, but installed a new Chrysler hemi-powered engine in his car to race at Sebring in 1953, where he again retired, this time due to a rear suspension failure. Gregory's first win came in just his third race, in Stillwater, Oklahoma. Changing to a Jaguar, Gregory won several races in America, including the Guardsmans Trophy in Golden Gate Park, San Francisco and a race at Offutt Air Force Base in Omaha, Nebraska. At the end of 1953, Gregory was invited to his first international sports car race - the 1000 km Buenos Aires in Argentina, which he finished in 14th due to water pump problems.

Throughout 1954 and 1955, Gregory competed in European races, usually driving Ferraris. His record includes the Tourist Trophy at Dundrod and the 24 Hours of Le Mans (although his co-driver Mike Sparken retired before Gregory got a chance to drive). He also won the inaugural Nassau Trophy at the Bahamas Speed Week in 1954. Moving back to America in 1956, Gregory entered several SCCA races, often winning. In 1957, he had another attempt at the Argentine 1000 km race, this time winning. This performance got him a drive with Guglielmo Dei's Scuderia Centro Sud, a privateer Formula One team using the Maserati 250F. His first race was the 1957 Monaco Grand Prix, where he scored an impressive third-place finish, the first podium for an American in an F1 Grand Prix. He followed this with a string of good results, coming eighth in the German Grand Prix, and fourth in both the Pescara and Italian Grands Prix. Despite only competing in half of the races, Gregory ended the 1957 season in sixth place in the championship.

Gregory only competed in four Grands Prix in the 1958 season, due to injuries sustained through one of his trademark bailouts when his car was set to crash, this time in a sports car race at Silverstone in England. He did manage a fourth place at the Italian Grand Prix, and a 6th in the last race of the year, this Moroccan Grand Prix. Moving to Cooper-Climax for the 1959 season alongside Jack Brabham and Bruce McLaren, he scored two podium finishes - a third place at the Dutch Grand Prix, and a career-best second at the Portuguese Grand Prix. However, he missed the final two races of the season, again due to injuries sustained jumping from a car moments before it crashed. He finished eighth in the Championship, and with teammate Brabham winning the World Championship, Cooper won their first Constructor's Championship. Gregory scored a pole position and set a course record at the non-Championship race at Aintree, but his contract with Cooper was not renewed for the following year.

Gregory's early years of competition were marked by lots of crashes, often the result of pushing sub-par machinery past its ability. He flipped a thankfully rollbar-equipped Maserati at the Venezuelan Grand Prix in 1957, totalled two sports cars in 1958, and another two in 1959 (a Lister-Jaguar and a Tojeiro-Jaguar). In the latter of these incidents he broke his leg and shoulder, keeping him away from his Formula 1 commitments. In 1960, trying to qualify an outdated Cooper-Maserati at Nürburgring he went off the track and was thrown clear of the car. After this period, however, his driving style matured and he began to develop a reputation as an elegant and careful driver.

Gregory continued in Formula One until 1965, but mainly with uncompetitive independent teams. He was unable to reproduce the results he obtained early in his career, his best being a 6th at the 1962 United States Grand Prix at Watkins Glen with the UDT Laystall team, in a Lotus 24. Running fourth, just behind eventual winner Dan Gurney at the French Grand Prix, Gregory retired with ignition problems, losing possibly his best chance at a maiden Grand Prix victory. Gregory did manage a win in the non-Championship 1962 Kanonloppet race at Karlskoga in Sweden, but this race did not feature any top teams.